Recently, lithium secondary batteries, which are chargeable/dischargeable and lightweight and have high energy density and high output density, have been widely used as energy sources for wireless mobile devices. Lithium secondary batteries have also attracted considerable attention as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), and electric vehicles (EVs), which have been developed to solve limitations such as air pollution and green-house gases that are caused by existing internal combustion engine vehicles that use fossil fuels such as gasoline and diesel vehicles.
Lithium secondary batteries are classified by electrode type into lithium ion batteries that use liquid electrolyte and lithium polymer batteries that use polymer electrolyte. Further, lithium secondary batteries are classified into cylindrical-type, prismatic-type, or pouch-type lithium secondary batteries according to the shape of their casing in which an electrode assembly is accommodated.
Among these, the pouch-type lithium secondary battery has a pouch exterior including a metallic layer (foil) and a multi-layered synthetic resin film which are applied to top and bottom surfaces of the metallic layer. Thus, the pouch-type lithium secondary battery may be developed as a lightweight lithium secondary battery and changed into various shapes because it is possible to significantly reduce the weight of the battery in comparison with the cylindrical-type or prismatic-type secondary lithium secondary battery which utilizes a metallic can.
The pouch exterior includes upper and lower exteriors which are formed by folding a middle portion of one side of a rectangular casing in a longitudinal direction. Here, press processing may be performed on the lower exterior to form a space part for accommodating an electrode assembly. Various electrode assembles having a structure, in which a cathode, a separator, and an anode that have mainly plate shapes are stacked, are accommodated in the space part of the lower exterior. Then, an electrolyte is injected, and edges around the space part of the lower exterior are closely attached to edges of the upper exterior corresponding to the lower exterior. Thereafter, the closely attached portions are thermally welded to form the sealed pouch-type secondary battery.
FIG. 1 is a schematic exploded perspective view illustrating a general structure of a representative pouch-type secondary battery according to a related art.
Referring to FIG. 1, a pouch-type secondary battery 1 includes an electrode assembly 10, electrode tabs 31 and 32 extending from the electrode assembly 10, electrode leads 51 and 52 welded to the electrode tabs 31 and 32, and a battery case 20 accommodating the electrode assembly 10.
The electrode assembly 10 may be a power generation device in which a cathode and an anode are successively stacked with a separator therebetween. The electrode assembly 10 has a stacked or stacked/folded type structure. The electrode tabs 31 and 32 extend from electrode plates of the electrode assembly 10, respectively. The electrode leads and 52 are electrically connected to the plurality of electrode tabs 31 and 32 respectively extending from the electrode plates through welding, respectively. Here, a portion of each of the electrode leads 51 and 52 is exposed to the outside of the battery case 20. Also, an insulation film 53 may be attached to a portion of each of top and bottom surfaces of the electrode leads 51 and 52 to enhance sealability and secure electrical insulation with respect to the battery case 20.
Also, the plurality of cathode and anode tabs 31 and 32 are integrally coupled to form welded portions, respectively. Thus, an inner, upper end of the battery case 20 is spaced a predetermined distance from a top surface of the electrode assembly 10, and each of the tabs 31 and 32 of the welded portions is bent in an approximately V shape (hereinafter, coupled portions of the electrode tabs and the electrode leads are called V-forming portions 41 and 42). The battery case 20 is formed with an aluminum laminate sheet and provides a space for accommodating the electrode assembly 10. Also, the battery case 20 has an overall pouch shape. After the electrode assembly 10 is built in an accommodation part of the battery case 20, and then an electrolyte (not shown) is injected, outer circumferential surfaces at which an upper laminate sheet and a lower laminate sheet of the battery case 20 contact each other are thermally welded to manufacture the secondary battery.
With respect to secondary batteries, there is a growing demand for medium- and large-sized batteries for vehicles and energy storage, and the medium- and large-sized batteries tend to be continuously thicker as large and high-energy cells are developed. Also, since the number of parallel connections of the battery must be reduced to obtain a cost reduction effect (in terms of module, pack assembly, and cell price), an increase in the capacity of cells tends to be accelerated.
However, with respect to a typical large-sized polymer battery, since the number of cell stacks constituting an electrode assembly may be high, an overall thickness may increase, and only a battery having a predetermined thickness (5 mm) or less may be prepared due to the forming limit of a pouch.